1. Field of the Invention
The present invention relates to image forming apparatuses and methods for forming an image based on a digital image. In particular, the invention relates to an image forming apparatus and an image forming method, for forming a high-quality image by controlling exposure energy density.
2. Description of the Background Art
With the advent of an age of digital information, demand is growing for printers, facsimiles, copiers and the like that are based on digital processing system. There is also a growing demand on these image forming apparatuses for enhanced image quality. Especially, development of copiers and printers is requested these days that are superior in reproducibility which enables high-definition images of various document fonts or photo level to be reproduced accurately.
However, it has been known the reproducibility varies depending on the type of images (difference of dot density) even if the condition of exposure is unchanged. A problem then arises that it is difficult to maintain a superior reproducibility for various images.
For example, suppose that the resolution of a line pattern consisting of lines on every second lines (hereinafter referred to as xe2x80x9cperiodic line patternxe2x80x9d) is ensured. Then, the diameter of a dot included in a one-dot pattern consisting of nonadjacent dots with a low dot density (hereinafter referred to as xe2x80x9cisolated dot patternxe2x80x9d) would be smaller than a desired value, or the dot itself would not be formed in some cases. On the contrary, if the dot diameter of the isolated dot pattern is ensured, the width of a line of the line pattern would be greater than a desired value, or the lines disappear resulting in solid black image.
This phenomenon is now described in conjunction with the drawings. FIG. 6 illustrates one example of digital image information. Here, the reference characters A to G and numerals 1 to 20 are applied for indicating dot positions.
Referring to FIG. 6, a virtual pattern in a memory space is shown consisting of rows (A, B, C, . . . ) and columns (1, 2, 3, . . . ). For example, the black portions represented by (row, column)=(A, 1) and the like mean xe2x80x9cprintxe2x80x9d (voltage level is high), while white portions represented by (row, column)=(B. 1) and the like mean xe2x80x9cnon-printxe2x80x9d (voltage level is low).
If data is read in one-dimensional manner from the memory storing the image information shown in FIG. 6 and a semiconductor laser is driven based on the read data, the laser is turned on (driven) when the read data is xe2x80x9cpintxe2x80x9d (voltage level is high) and the laser is not turned on (driven) when the read data is xe2x80x9cnon-printxe2x80x9d (voltage level is low).
Specifically, xe2x80x9cAxe2x80x9d is first designated as a row address which is one of signals input to the memory and then xe2x80x9c1, 2, 3, . . .xe2x80x9d are designated in this order as a column address which is another input signal. Accordingly, row data in row A thus designated are successively read and a laser driver which receives the data controls turning on/off of the laser. In this way, an electrostatic latent image according to the image information regarding row A is formed on a photoreceptor.
Next, xe2x80x9cBxe2x80x9d is designated as a row address and xe2x80x9c1, 2, 3, . . .xe2x80x9d are successively designated as a column address. Then, the designated data regarding row B are also read and the laser driver controls turning on/off of the laser according to the data. An electrostatic latent image is thus formed based on the image information for row B as done for row A.
This operation is repeated to form on the photoreceptor a two-dimensional electrostatic latent image pattern corresponding to the image pattern shown in FIG. 6.
FIGS. 11 and 12 respectively illustrate extreme results of development obtained by performing such a process of forming an electrostatic latent image for each of all image patterns shown in FIG. 6 under the same exposure condition.
FIG. 12 shows a result obtained by forming an image under an exposure condition which enhances the reproducibility of the line pattern of the original image (FIG. 6). Under this exposure condition, the line pattern is properly reproduced as shown in FIG. 12 while the reproducibility of the isolated dot pattern is deteriorated. In an extreme case, the isolated dot disappears or the dot is not reproduced at all.
FIG. 11 shows a result obtained by forming an image under an exposure condition which ensures the dot diameter of the isolated dot pattern of the original image (FIG. 6). Under this exposure condition, the isolated dot is appropriately reproduced with a desired dot diameter as shown in FIG. 11 while the reproducibility of the line pattern is impaired. In an extreme case, the line width increases to cause non-print portions to disappear, resulting in a solid black image.
In order to solve this problem that a superior reproducibility cannot be ensured for images having different dot densities, Japanese Patent Laying-Open No. 63-64763 discloses a method according to which print data itself is corrected (related art 1). Specifically, an isolated one-dot print data is detected from print data, and one bit preceding or following the detected one-dot print data is corrected as print data.
Japanese Patent Laying-Open No. 63-296069 discloses a method for solving that problem by changing the diameter of a beam spot on the photoreceptor (related art 2). Specifically, an isolated one-dot print data is detected from print data, and the diameter of a beam spot for the detected one-dot print data is increased.
This problem that a superior reproducibility cannot be ensured for images having different dot densities should be considered together with an influence of change in the film thickness of the photoreceptor as time progresses. Reproducibility of an image is considerably affected by charging and light attenuation characteristics of the photoreceptor as well as modulation transfer function of electric field within the photoreceptor, and the like. These characteristics of the photoreceptor vary depending on the film thickness of the photoreceptor,
The film thickness of the photoreceptor decreases with time due to contact with a cleaning member for removing residual toner, friction with a paper for transfer, and the like. Therefore, the various characteristics of the photoreceptor change with time merely by using the image forming apparatus.
A problem then arises that the density of a black portion of a printed image or the brightness of the image changes from the initial setting, or the reproducibility of images having different dot densities cannot be maintained.
Various image formation techniques have been proposed in order to overcome this problem, considering the change with time in the film thickness of the photoreceptor. As representative approaches, Japanese Patent Laying-Open No. 8-95433 discloses a technique of ensuring the brightness of an image by sensing change in the film thickness of the photoreceptor to control the amount of exposure lamp (related art 3), Japanese Patent Laying-Open No. 5-16533 discloses a technique of ensuring the density by measuring the optical density of a reference patch image formed on the photoreceptor to feed back the result of the measurement (related art 4), and Japanese Patent Laying-Open No. 11-15214 discloses a technique of controlling the charging potential of the photoreceptor and controlling the development bias potential in consideration of variation in the image density due to change in the development electric field caused by change with time in the photoreceptor characteristics (related art 5).
However, with the higher image resolution, it is more difficult to ensure a superior reproducibility for images having different dot densities. If an image having a resolution of 1200 dpi is to be formed by using a practically employed photoreceptor with a film thickness of 20 xcexcm to 30 xcexcm, for example, a superior reproducibility for images with different densities cannot be ensured by the techniques discussed above.
When the same exposure energy density is used to form images with a low dot density and with a relatively high dot density respectively, respective latent images formed on the photoreceptor are different from each other in the latent image potentials of print portions and non-print portions.
Specifically, the image having a high dot density has a portion on the photoreceptor where exposure lights overlap. Therefore, on a negatively charged photoreceptor, for example, the portion where exposure lights overlap has a potential which is greater than a normal potential by the extra exposure energy. In other words, the effect of overlapping potential is observed.
This overlapping effect can be reduced by optimizing the exposure spot diameter as disclosed in related art 2 if resolution is low. However, if the resolution is high, there are more portions where the overlapping effect occurs, and thus it is impossible to weaken the overlapping effect just by changing the exposure spot diameter.
Even if the technique of changing the contents of original print data is used as disclosed in related art 1, the original image could not be reproduced precisely. Therefore, related art 1 cannot fundamentally solve this problem.
Related arts 3 to 5 disclose solutions for the variation in the density and the brightness of an image caused by the change with time in the film thickness of the photoreceptor. However, any solution cannot ensure the reproducibility of image patterns having different dot densities.
Accordingly, even if the exposure unit is controlled only in consideration with the influence of change in the film thickness of the photoreceptor, an output image cannot be formed which is truly superior in the reproducibility.
One object of the present invention is to provide an image forming apparatus and a method of forming an image which can provide a superior image reproducibility for images having various dot densities.
Another object of the invention id to provide an image forming apparatus and a method of forming an image which can ensure an image reproducibility over a long period of time for images having various dot densities even if the film thickness of a photoreceptor changes.
Those objects of the invention are achieved by an image forming apparatus including following elements. Specifically, according to one aspect of the invention, the image forming apparatus includes an exposure unit for exposing a photoreceptor which is charged to a predetermined potential to light, a control unit for controlling exposure energy density of the exposure unit based on image information, and a development unit for visualizing an electrostatic latent image formed on the photoreceptor by the exposure unit. The control unit includes a first storage unit storing a plurality of reference images, a comparison unit comparing the image information with a reference image, and a first determination unit determining an exposure energy density corresponding to each of a plurality of pieces of pixel information constituting the image information based on result of the comparison by the comparison unit.
According to the present invention, an image forming apparatus can be provided which exhibits a superior image reproducibility for images with various dot densities.
Preferably, the control unit of the image forming apparatus further includes a second storage unit storing an exposure energy density corresponding to each reference image. The first determination unit includes a second determination unit which determines, based on result of the comparison by the comparison unit, an appropriate one of the plurality of reference images for each of the plurality of pieces of pixel information. Accordingly, an exposure energy density corresponding to each pixel information is determined based on the determined reference image and the stored exposure energy density.
The exposure energy density for each pixel information can thus be determined easily by referring to the second storage unit based on a reference image associated with each pixel information.
Still preferably, the comparison unit of the image forming apparatus includes an extraction unit extracting from the image information a matrix image formed of predetermined units for each pixel information, and a decision unit deciding whether or not the extracted matrix image matches a reference image. The second determination unit includes a first recognition unit which recognizes, if the extracted matrix image matches the reference image according to the decision by the decision unit, this reference image as an appropriate reference image.
In this way, if a matrix image matches a reference image, an optimum exposure energy density can immediately be determined for each pixel information.
Still preferably, the second determination unit includes a second recognition unit which recognizes, if the extracted matrix image does not match the reference image according to the decision by the decision unit, a reference image similar to the extracted matrix image as an appropriate reference image.
Accordingly, even if a matrix image does not match a reference image, a reference image similar to that matrix image is determined so as to eliminate the need for storing a large number of reference images.
Preferably, the image forming apparatus further includes a film thickness sensing unit sensing the film thickness of the photoreceptor. The first determination unit determines an exposure energy density corresponding to each pixel information in consideration of result of the sensing by the film thickness sensing unit.
Accordingly, an image forming apparatus can be provided which can ensure an image reproducibility for images with various dot densities over a long period of time even if the film thickness of the photoreceptor changes.
Preferably, the image forming apparatus further includes a film thickness sensing unit sensing the film thickness of the photoreceptor. The second storage unit stores an exposure energy density corresponding to each reference image in association with the film thickness of the photoreceptor, and the first determination unit determines an exposure energy density corresponding to each pixel information based on the determined reference image and the stored exposure energy density associated with the film thickness of the photoreceptor.
In this way, exposure energy density can easily be determined that is associated with the film thickness of the photoreceptor.
According to another aspect of the invention, a method of forming an image includes the steps of exposing a photoreceptor charged to a predetermined potential to light, controlling exposure energy density of an exposure unit based on image information, and developing an electrostatic latent image formed in the exposing step on the photoreceptor to visualize the latent image. The controlling step includes a first storage step of storing a plurality of reference images, a second storage step of storing an exposure energy density corresponding to each reference image, extracting from the image information a matrix image constituted of predetermined pixel units for each pixel information, deciding if the extracted matrix image matches any of the reference images, a first determination step of determining an appropriate one of the reference images for each of a plurality of pieces of pixel information constituting the image information based on result of the decision in the deciding step, and a second determination step of determining an exposure energy density corresponding to each pixel information based on a determined reference image and stored exposure energy density.
According to the present invention, it is possible to provide a method of forming an image exhibiting a superior image reproducibility for images with various dot densities.
Preferably, the image forming method further includes the step of sensing the film thickness of the photoreceptor. In the first storage step, an exposure energy density corresponding to each reference image is stored in association with the film thickness of the photoreceptor, and in the second determination step, an exposure energy density corresponding to each pixel information is determined based on a determined reference image and the exposure energy density stored in association with the film thickness of the photoreceptor.
In this way, it is possible to provide an image forming method ensuring an image reproducibility for images with various dot densities over a long period of time even if the film thickness of the photoreceptor varies.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.